1. Field of the Invention
The present invention relates to a method of registering an unregistered optical network unit (ONU) in a telecommunications network. The present invention also relates to a telecommunications network comprising an optical line terminal (OLT) including a plurality of optical subscriber units (OSUs) and an ONU connected to the OLT. Furthermore, the present invention relates to an ONU for use in communication through a connection to an OLT.
2. Description of the Background Art
A telecommunications network which links a communication station possessed by a communication common carrier to customer premises may be called as an access network. In recent years, with the trend that communications capacities have increased, optical access networks enabling huge amounts of information to be transmitted by means of optical communications are becoming the mainstream of access networks.
As a form of optical access network, passive optical networks (PONs) are available. The PON is configured to have an OLT installed in a communication station, plural ONUs installed on respective subscriber premises and an optical splitter. The OLT and ONUs are interconnected to each other via the optical splitter with optical fiber lines.
The interconnection between an OLT and an optical splitter can be implemented by a single-core optical fiber, which is shared by plural ONUs. Optical splitters are, in general, inexpensive passive elements, so that PONs are superior in economics and in maintenanability. For those reasons, PONS are being rapidly introduced.
In a PON, signals transmitted by ONUs toward an OLT, which may be referred to as upstream optical signals, are first combined by an optical splitter and then transmitted to the OLT. On the contrary, signals transmitted by the OLT toward the ONUs, which may be referred to as downstream optical signals, are first split by the optical splitter and then transmitted to the respective ONUs. In order to prevent upstream and downstream optical signals from interfering with each other, the upstream and downstream optical signals are assigned to wavelengths that are different therebetween.
PONs are implemented by a variety of multiplexing techniques, such as time division multiplexing (TDM) assigning short time slots to subscriber terminals, wavelength division multiplexing (WDM) assigning different wavelengths specific to subscriber terminals, code division multiplexing (CDM) assigning different codes specific to subscriber terminals. In those multiplexing techniques, PONs based on the TDM, i.e. TDM-PON, currently enjoy the widest acceptance. Among TDM-PONs, time division multiple access (TDMA) is prevailing, in which an OLT manages timing at which ONUs transmit upstream optical signals so as to prevent upstream signals transmitted from the different ONUs from colliding with each other.
One of the PON systems based on Ethernet (trademark) may be referred to as Ethernet-PON, which includes a PON system based upon Gigabit (1×109 bit/sec) Ethernet, that is, GE-PON. The GE-PON is standardized, for example, by IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3ah and IEEE 802.3av.
In order to establish communications between an OLT and ONUS in a GE-PON system, it is necessary to register the ONUS in the OLT. Generally in GE-PON systems, the single OLT has plural ONUS connected thereto. Thus, when an ONU not yet registered in the OLT is newly registered in the OLT, the registration has to be performed without affecting communications between the OLT and other ONUs already registered. For that aim, the IEEE 802.3ah and IEEE 802.3av Standards regulate a procedure of detecting an unregistered ONU and then registering that OLT. The procedure may be referred to as a discovery sequence.
In the discovery sequence, an OLT periodically broadcasts a discovery gate massage notifying all ONUS of transmitting timing. The discovery gate message is transmitted to every ONU, regardless of whether or not the ONU is already registered in the OLT. A time interval at which a discovery gate message is transmitted from the OLT may be referred to as a discovery period. After an ONU newly connected to the PON system, although not registered, is powered on and then prepared for reception of signals, it will periodically receive discovery gate messages.
If unregistered one of the ONUS receives a discovery gate message, it transmits a registration request to the OLT to request that the ONU be registered in the OLT. The registration request contains a media access control (MAC) address, which is the identification of the ONU.
In the OLT, a discovery window is set. The OLT opens a discovery window, in which it waits for reception of a registration request from an unregistered ONU.
The OLT receives a registration request to thereby recognize the MAC address of the unregistered ONU. The OLT in turn transmits a register message, which contains a logical link identifier (LLID) on the PON system, to the ONU having the recognized MAC address.
Subsequently to the transmission of the register message, the OLT transmits a gate message notifying the ONU of a transmission bandwidth and timing and allowing the ONU to transmit upstream optical signals.
On receiving the gate message, the unregistered ONU transmits a register acknowledgement (ACK) to the OLT. When the OLT receives the register ACK, registration of the new ONU is completed, namely, the discovery sequence thus finishes. After the new ONU is registered, normal communications between the OLT and the newly registered ONU will start.
At the present time, a PON system based upon both TDM and WDM, i.e. TDM/WDM-PON, is proposed. In the TDM/WDM-PON, an OLT may include a plurality of optical subscriber units (OSUs).
In the TDM/WDM-PON, OSUs are assigned to transmission wavelengths that are different OSU by OSU. The OSUs transmit downstream optical signals on the transmission wavelengths specifically assigned thereto. On the contrary, the ONUS transmit upstream optical signals on the transmission wavelengths and timing notified on the downstream optical signal by the OSU.
In the TDM/WDM-PON, ONUS are grouped correspondingly to plural OSUs so that each ONU is to be managed dedicatedly by either one of the plural OSUs, thus reducing the number of ONUs managed by each OSU. This can expand the service bandwidths provided to subscriber units.
In the TDM/WDM-PON thus configured, it is sufficient for each ONU to be registered in either one of the plural OSUs. Furthermore, as described previously, in the TDM/WDM-PON each ONU communicates at a wavelength appropriate for an OSU associated therewith. It is thus preferable for the ONUS to have the wavelengths thereof variably available for transmission and reception.
As disclosed in Japanese patent laid-open publication No. 2011-004270 to Fujiwara et al., an ONU in the TDM/WDM-PON includes a variable-wavelength (VW) filter. In this prior art system, the ONU, when unregistered, sweeps the passable, or transmissive, wavelength of its VW filter in order to wait for receipt of a discovery gate message.
When the unregistered ONU receives a downstream optical signal, it measures the optical power of the optical signal. When the measured optical power is less than a predetermined value, or threshold, the VW filter shifts its transmissive wavelength to a different wavelength.
On the contrary, when the measured optical power is equal to or greater than the predetermined value, the unregistered ONU makes a decision as to whether or not the downstream optical signal is a discovery gate message. If a discovery gate is detected in the optical signal, a discovery sequence will proceed as the processes described above.
If no discovery gate is detected, the ONU waits for receipt of a discovery gate message for a predetermined period of time, i.e. wait time. If no discovery gate message is received during the period of time, namely, the period of time is over, then the VW filter shifts its transmissive wavelength again to another different wavelength.
In the unregistered ONU, the VW filter repeats the shift of its transmissive wavelength until it receives a discovery gate message, thus searching for a transmissive wavelength on which it can receive a discovery gate message.
A wait time during which an unregistered ONU awaits a discovery gate message at a transmissive wavelength of its VW filter is set longer than a discovery period. That is, the wait time TT-FL of an unregistered ONU at each transmissive wavelength with respect to a discovery period TDIS is set by following Expression (1):TT-FL>TDIS  (1)
The condition defined by Expression (1) ensures that a discovery gate message is sent at least once within the wait time at each transmissive wavelength. This makes it possible to correctly determine, at each transmissive wavelength set, whether or not a discovery gate message can be received. The VW filter thus sequentially, e.g. incrementally or decrementally, switches its transmissive wavelength, the unregistered ONU thereby being able to reliably receive a discovery gate message.
In the TDM/WDM-PON, the discovery period TDIS is usually set equal to a dynamic bandwidth allocation (DBA) period multiplied by a constant α, where α is greater than unity. The DBA period is a period of time in which an OSU transmits a bandwidth allocation signal, such as a gate message in the aforementioned IEEE Standards, notifying an ONU of a transmission bandwidth and timing. The constant α is a value set in advance as a parameter during the operation of the system. Hence, when the DBA period is rendered to change, the discovery period TDIS is changed accordingly.
When the discovery period TDIS is varied, the wait time TT-FL of the unregistered ONU at each transmissive wavelength may become shorter than the discovery period TDIS. In this case, a relation given by following Expression (2) is developed between the wait time TT-FL and the discovery period TDIS:TT-FL<TDIS  (2)
Under the condition given by above Expression (2), the transmissive wavelength of a VW filter is changed at a transmission interval at which a discovery gate message is sent. That may cause a condition to continue in which the wavelength of a discovery gate is not coincident with the transmissive wavelength of a VW filter. Consequently, it is highly likely that it takes a long time until a discovery sequence commences.
In order to maintain the condition defined by Expression (1) whenever the DBA period changes, the OSU would be adapted to notify the ONUS of the change of the DBA period. However, the ONU, when not activated, could not receive a notice of the change of the DBA period. Therefore, the ONU, when started up, would not set the wait time TT-FL appropriate for the change of the DBA period.
In order to maintain the relation given by Expression (1), a maximum assumption method would be introduced in which the possible maximum DBA period is considered so as to set the wait time TT-FL of an unregistered ONU at each transmissive wavelength. In the maximum assumption method, an unregistered ONU would calculate the discovery period TDIS on the basis of the maximum value of the DBA period. Then, the wait time TT-FL would be set so that Expression (1) could hold at the calculated discovery period TDIS. Thence, if the DBA period is changed, the wait time TT-FL would be prevented from becoming shorter than the discovery period TDIS. The relation given by Expression (1) would thus be maintained.
In the maximum assumption method, however, the wait time TT-FL would have to be set to a large value, which would require an extensive time until the wait time expires. Accordingly, when the DBA period is not maximal, if the wavelength of a discovery gate message is not coincident with the transmissive wavelength of the VW filter, it would then be highly likely that discovery gate messages the unregistered ONU fails to receive would repetitively be transmitted. For this reason, the unregistered ONU would inefficiently receive discovery gate messages, and hence it would take a long time until the discovery sequence is started and completed.